Acrylic acid or methacrylic acid (hereafter they are referred to as “(meth)acrylic acid” collectively) is one used as a production raw material of industrial products, and a chemical substance mass produced in a large scale plant. In general, these compounds are produced via the step for obtaining a solution of (meth)acrylic acid by separation of non-condensable gas from a crude product, or further by the various purification steps, to obtain high purity products.
For example, in the step for producing acrylic acid, propylene, propane, acrolein or the like is subjected to catalytic vapor phase oxidation by molecular oxygen-containing gas in the presence of an oxidation catalyst, by which byproducts of substances with low boiling point are generated, such as acetic acid, lower aldehyde, water and the like; and substances with high boiling point such as furfural, maleic anhydride and the like, other than acrylic acid which is an objective product. Therefore, the product is obtained by introducing the resulting mixed gas to a separation apparatus (for example, an absorption tower) of non-condensable gas, and by subjecting to condensation or contacting with an absorbing agent for acrylic acid to obtain a solution containing acrylic acid and other byproducts, and by separation and purification of acrylic acid from this solution by a method such as distillation, diffusion, extraction, crystallization or the like.
In this way, as a method for producing (meth)acrylic acid having the step for obtaining a (meth)acrylic acid solution by introducing (meth)acrylic acid-containing gas obtained by catalytic vapor phase oxidation of propylene etc. to a separation apparatus (absorption tower), and subjecting the gas to contact with an absorbing agent for acrylic acid, there are for example, technologies disclosed in (A) U.S. Pat. No. 7,067,696B2, (B) US2006/0211886A1, (C) U.S. Pat. No. 6,667,419B2, (D) U.S. Pat. No. 5,780,679A, (E) U.S. Pat. No. 7,258,766B2, (F) U.S. Pat. No. 6,825,381B2 and (G) U.S. Pat. No. 7,183,428B2. In these Patent Literatures, it is described that (meth)acrylic acid-containing gas is cooled before being supplied to a capturing tower, which is a kind of a separation apparatus of non-condensable gas. For example, in Patent Literature (C), it is described that (meth)acrylic acid-containing gas comes out from a reactor at from 200 to 350° C., and supplied to an absorption tower, which is a kind of a separation apparatus of non-condensable gas, at from 100 to 300° C.
These methods have each characteristics, in such a way that as for technology of Patent Literature (A), temperature of mixed gas at the suction port of an compressor is specified, to suppress clogging or the like of the compressor of molecular oxygen-containing gas to be used in a catalytic vapor phase oxidation reaction. As for technologies of Patent Literatures (B) and (C), as they are for suppressing clogging of a separation apparatus of non-condensable gas, a method in which (meth)acrylic acid-containing gas is supplied from a plurality of places to a separation apparatus, and a method in which filling substances with different absorption efficiency are set inside a separation apparatus in multiple stages, respectively, are disclosed.
In addition, as for technology of Patent literature (D), energy is removed from a separation apparatus of non-condensable gas in order to solve a problem of entrainment of acrylic acid by discharged gas. Also as for technology of Patent literature (E), mass fraction of acrylic acid in acrylic acid-containing gas is specified in order to reduce concentration of acrylic acid remained in discharged gas.
However, in these technologies, there is no consideration on fluctuation of concentration of a (meth)acrylic acid solution obtained from a separation apparatus of non-condensable gas. On the other hand, there is technology described in Patent Literature (F), as technology for controlling concentration of a (meth)acrylic acid solution, in view of the fact that fluctuation of an amount of water contained in a (meth)acrylic acid solution makes difficult to secure operation service stability in the next and the subsequent steps. Such technology has been proposed in considering the fact that variation of the amount of water contained in gas discharged from a reactor, depending on fluctuation of atmosphere condition, results in fluctuation of concentration of a (meth)acrylic acid solution, and is technology to control concentration of (meth)acrylic acid by changing the amount of water in gas discharged from the tower top by means of controlling temperature or pressure at the tower top of a capturing tower. However, in this literature, there is no disclosure on relation between the temperature of gas discharged from a reactor, and the concentration of the acrylic acid solution.
In addition, a high concentration of a (meth)acrylic acid solution obtained from a separation apparatus of non-condensable gas, enhances efficiency in the following purification step. Therefore, technology to increase the concentration has also been developed. Patent Literature (G) discloses technology for subjecting an acrylic acid solution obtained in the capturing tower to the crystallization step and the distillation step, and then subjecting the resulting distillate to circulate into the capturing tower. By such technology, an acrylic acid solution with a high concentration of equal to or higher than 80% by volume can be obtained. However, although an acrylic acid solution with high concentration can be obtained according to the technology, the technology is not aimed at obtaining an acrylic acid solution in constant concentration stably.
As described above, technology to capture (meth)acrylic acid efficiently by separation of non-condensable gas from (meth)acrylic acid-containing gas, which is obtained by a catalytic vapor phase oxidation reaction, has been known conventionally. However, it was found that these prior arts raised a problem when they are applied to obtaining a (meth)acrylic acid solution with high concentration stably from a separation apparatus of non-condensable gas.
Namely, as described also in prior arts, temperature range of (meth)acrylic acid-containing gas obtained by a catalytic vapor phase oxidation reaction is, for example, from 200 to 350° C., which exceeds 100° C. This is because, although the reaction proceeds even at low temperature as long as a catalyst to be used maintains sufficient activity, the reaction temperature is forced to be raised to maintain production amount of (meth)acrylic acid in response to deterioration of the catalyst, resulting in increase in also temperature of the resulting (meth)acrylic acid-containing gas. In addition, in the step for obtaining a (meth)acrylic acid solution by separation of non-condensable gas from (meth)acrylic acid-containing gas, compounds having low boiling point, such as water and the like continue to evaporate. Therefore, in response to temperature or water content of the (meth)acrylic acid-containing gas, concentration of the resulting (meth)acrylic acid solution fluctuates. To solve this problem, in general, such measures are taken that the concentration of a (meth)acrylic acid solution is maintained constant by controlling temperature of the separation apparatus, by using a method that, for example, a part of the resulting (meth)acrylic acid solution is circulated to a separation apparatus of non-condensable gas (an absorption tower), and the amount of the circulation solution to pass through a cooler installed at a circulation line or the like is controlled before the circulation solution is circulated to the separation apparatus.
Here, although it is manageable even by this embodiment in order to obtain a (meth)acrylic acid solution having concentration required conventionally, however, there were not-manageable cases in obtaining a solution having high concentration. For example, conventionally, concentration of a (meth)acrylic acid solution obtained from a separation apparatus of non-condensable gas (an absorption tower) is in a degree of from 50 to 70% by mass. In this case, because amount of steam discharged from the separation apparatus of non-condensable gas is low, and it is enough to actively cool the separation apparatus of non-condensable gas, control can be carried out easily. However, in the case where an object is to obtain a (meth)acrylic acid solution with higher concentration, subtle control is required in response to temperature variation of (meth)acrylic acid-containing gas which is introduced into the separation apparatus of non-condensable gas, which results in out of a controllable range by the separation apparatus of non-condensable gas, or forced to operate at the vicinity of limit of the controllable range, thus giving fluctuation of concentration of a (meth)acrylic acid solution, even by a very small external turbulence.
In more specifically, in the case of obtaining a (meth)acrylic acid solution having conventional concentration, provided that minimal heat removal amount in a (meth)acrylic acid solution to be circulated is 100, a maximal heat removal amount of about 150 may be enough. However, in the case of obtaining a (meth)acrylic acid solution having high concentration, because it is required to maintain temperature inside a separation apparatus relatively high to increase amount of water to be evaporated inside the separation apparatus of non-condensable gas, and extremely small amount of heat removal is necessary when gas temperature at the entrance is low, heat removal amount of a solution to be circulated is set to be about 26 to 140. Difference of heat removal amount in this case is about 5.4 times (140/26) as compared with a conventional case of 1.5 times (150/100), thus resulting in out of a controllable range by a method such as controlling amount of circulating solution passing through a cooler at the circulation line, which inevitably provides concentration fluctuation. On the other hand, in order to obtain more concentrated (meth)acrylic acid solution, the water discharge amount is necessary to be increased by means of increasing the temperature inside acrylic acid absorption tower or by decreasing the pressure, for example. For the means, simply temperature of the acrylic acid absorption tower can be set high. If high concentrated (meth)acrylic acid solution is aimed in the case of low temperature of the gas including (meth)acrylic acid, the heat removal amount in the acrylic acid absorption tower should be decreased. However, the conventional systems have only a means to cool the acrylic acid absorption tower to effectively capture (meth)acrylic acid. Then, in the case of low temperature of the gas including (meth)acrylic acid, the conventional systems have not been able to control the temperature of the separation apparatus of non-condensable gas by removing slight amount of heat to obtain (meth)acrylic acid solution as high concentrated as the case of high temperature. Namely, it is impossible to obtain a (meth)acrylic acid solution having high concentration stably by a conventional method.
Accordingly, it is an object of the present invention to provide a method for obtaining an acrylic acid solution having high concentration stably, regardless of temperature fluctuation of gas discharged from a reactor for catalytic vapor phase oxidation, in the step for producing (meth)acrylic acid by a catalytic vapor phase oxidation reaction.